Miami Air have used 738's on Reach flights for the US Mil. AmTrans 752's and 753's also operate transatlantic on a daily basis, as do Omni and North American with 752's. A number of UK and Canadian charter operators have also used 757's for Atlantic work.

Correct, they are also using A 319's for selected long-haul flights but in a two class configuration (Business and Economy Class) and not for transatlantic flights, the current destinations are: Kuwait, Pointe Noire, Riad, Malabo, and Tashkent. The flight to Doha has been discontinued lately because it was not profitable. The next Dedicate destination should be Kish Island but I think they don't have traffic rights at the moment.

Yes, all FI flights are operated with the 757 except the new flight to SFO, it is operated with a 767. If I remember correct did FI also fly with the 737 (-400?) to MSP a few years ago but I am not 100% sure.

Well... it seems fine to me that some carriers are doing transatlantic and long distance flights with the smaller single aisle planes. I just finished a 6 1/2 hour transcon on a B737-700 and it was quite pleasant... plenty of room, very good FA service... easy to board and deplane. Besides... you only have to look around to note that smaller jet aircraft are making a big impact in the market and one does not necessarily have to cross "the pond" in a huge twin aisle to be comfortable and safe, given the incredible reliability of modern aircraft and engines.
Aside from that.. there's the Europeans coming out with the A380 and having launched the Queen Mary 2, a 150+ thousand ton floating liner... I guess that a lot of folks do think that "size matters", ha ha...